1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a switching power converter control system and method with direct current transformer sensing.
2. Description of the Related Art
Switching power converters provide regulated output voltages for numerous electronic products including lamps, such as light emitting diode and gas discharge type lamps, cellular telephones, computing devices, personal digital assistants. As the name suggests, switching power converters include a switch that controls the output voltage. Sensing current in the switch provides feedback to a controller to allow the controller to control the switch and, thus, regulate the output voltage of the switching power converter. The switch current can be sensed in any number of ways. Switch current can be sensed as a voltage across a sense resistor in series with the switch. However, conducting a high switch current across the sense resistor of the switching power converter can result in consequential power losses. Current transformers provide a more efficient way of sensing the switch current. A primary-side of the current transformer is placed in series with the switch to conduct the switch current. The secondary-side conducts a stepped-down switch current. A sense resistor across the secondary-side of the current transformer develops a voltage corresponding to the stepped-down switch current, thus, lowering the power loss across the sense resistor.
FIG. 1 depicts a power control system 100, which includes switching power converter 102. In at least one embodiment, switching power converter 102 operates in a continuous conduction mode (CCM). In CCM, the inductor current iL in inductor 104 does not go to zero during operation of switching power converter 102. As subsequently described in more detail, in CCM operation, PFC and output voltage controller 106 senses switch current iDP in switch 108 by sensing a voltage VCTS corresponding to a current iDS across a secondary winding 110 of current transformer 112. Switch 108 is implemented as a field effect transistor (FET).
Switching power converter 102 converts voltage sourced from voltage source 114 into a regulated link voltage VLINK across link capacitor 116. Voltage source 114 supplies an alternating current (AC) input voltage VIN to a full bridge diode rectifier 117. The voltage source 114 is, for example, a public utility, and the AC voltage VIN is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. The rectifier 117 rectifies the input voltage VIN and supplies a rectified, time-varying, line input voltage VX to the switching power converter 102. Capacitor 115 filters high reference components of input voltage VX and, thus, provides electromagnetic interference (EMI) protection.
The switching power converter 102 includes at least two switching operations. The first operation controls switch 108 to provide power factor correction (PFC), and the second operation controls switch 108 to provide regulation of link voltage VLINK. The goal of power factor correction technology is to make the switching power converter 102 appear resistive to the voltage source 114 so that the real power provided to switching power converter 102 is equal to the apparent power provided to switching power converter 102.
During operation of switching power converter 102, the inductor current iL ramps ‘up’ when switch 108 conducts, i.e. is “ON”, and ramps down when switch 108 is nonconductive, i.e. is “OFF”. When switch 108 is ON, inductor current iL magnetizes inductor 104 and also magnetizes the primary-side winding 118 of current transformer 112. In at least one embodiment, current transformer 112 is configured as a toroid with a primary-side to secondary-side winding ratio of 1:N. “N” is, for example, 10. The primary-side transformer current iDP equals the inductor current iL when switch 108 is ON. Thus, the second-side current iDS equals iDP/N when switch 108 is ON.
When switch 108 is ON, secondary-side current iDS flows through diode 120 and is filtered by filter 122 to reduce noise. Resistor 121 provides a number of functions, such as reverse current protection for diode 120. The resistance value of resistor 121 is a matter of design choice. In at least one embodiment, resistor 121 has a resistance greater than an input resistance of load 132. In at least one embodiment, the resistance of resistor 121 is approximately equal to at least ten times the input resistance of load 132. A secondary-side current transformer voltage VCTS develops across filter 122 when switch 108 is ON. In embodiment, filter 122 is designed so that the transformer voltage VCTS is 1V at the peak of secondary-side current iDS. Current synthesizer 126 charges capacitor 128 with a current proportional to the switch current iDS. The transformer voltage VCTS is level shifted up one ‘diode drop’ voltage across diode 124 relative to the voltage VCTC across capacitor 128. (A “diode drop” voltage refers to a voltage drop across a forward biased diode. A typical diode drop voltage is 0.7V). The inductor current iL is the same as the switch current iDS when switch 108 is ON. Thus, control signal generator 130 determines the inductor current iL when switch 108 is ON based on the secondary-side voltage VCTS.
When switch 108 is OFF, secondary-side current iDS is zero, and inductor current iL flows through diode 127 to charge link capacitor 116. Diode 127 prevents reverse current flow into inductor 104. PFC and output voltage controller 106 includes a current synthesizer 128 to reconstruct the inductor current iL during the OFF time of switch 108. In general, current synthesizer 126 controls the discharge of capacitor 128 in proportion to the input voltage VX and the link voltage VLINK to reconstruct the down slope of inductor current iL and, thus, determine the inductor current iL when switch 108 is OFF. Upon determining the switch current iDS while switch 108 is ON using the secondary-side voltage VCTS of current transformer 110 and reconstructing the inductor current iL when switch 108 is OFF, control signal generator 130 generates control signal CS0 to regulate the link voltage VLINK and provide power factor correction for switching power converter 102. The link voltage VLINK is supplied to load 132. A more complete explanation of an exemplary operation of PFC and output voltage controller 106 is described in “High Performance Power Factor Preregulator”, Unitrode Products from Texas Instruments, part numbers UC2855A/B and UC3855A/B, Data Sheet June 1998, revised October 2005 and Application Report SLUA146 May 1996, revised April 2004.
The cost of external components is generally higher than the cost of integrated components. For example, if filter 122 could be eliminated, the cost of power control system 100 could potentially be reduced.